Strain wave gearings having cylindrical externally toothed gears are typically provided with a stationary-side internally toothed gear secured so as not to rotate, a wave generator that is a rotation-inputting element, a drive-side internally toothed gear that is a reduced-rotation-outputting element, and a cylindrical externally toothed gear capable of flexing in the radial direction and meshing with the stationary-side internally toothed gear and drive-side internally toothed gear. In typical strain wave gearings, the externally toothed gear is caused to flex into an ellipsoidal shape, the ellipsoidally flexed externally toothed gear meshing with the stationary-side and drive-side internally toothed gears at both end positions along the major axis of the ellipsoidal shape.
Patent Documents 1 and 4 disclose typical strain wave gearings in which the number of teeth of the stationary-side internally toothed gear is two greater than that of the externally toothed gear, and the number of teeth of the drive-side internally toothed gear is equal to that of the externally toothed gear. In Patent Document 1, the external teeth of the externally toothed gear are bisected at the tooth-trace-direction central portion thereof, one of the external-tooth portions being capable of meshing with the stationary-side internally toothed gear, and the other of the external-tooth portions being capable of meshing with the drive-side internally toothed gear. Patent Document 4 indicates that the rim wall thickness of the externally toothed gear dramatically affects the tooth bottom fatigue strength of the externally toothed gear.
In the strain wave gearings disclosed in Patent Documents 1 and 4, when the wave generator rotates, the externally toothed gear rotates more slowly at a speed ratio corresponding to the difference in the number of teeth with respect to the stationary-side internally toothed gear. The reduced rotation of the externally toothed gear is outputted from the drive-side internally toothed gear, which rotates integrally with the externally toothed gear.
Patent Document 2 discloses a strain wave gearing in which the number of teeth of the stationary-side internally toothed gear is two greater than that of the externally toothed gear, and the number of teeth of the drive-side internally toothed gear is two less than that of the externally toothed gear. In this strain wave gearing, when the wave generator rotates, the externally toothed gear rotates more slowly at a speed ratio corresponding to the difference in the number of teeth with respect to the stationary-side internally toothed gear. The rotation of the externally toothed gear is increased at a speed ratio corresponding to the difference in number of teeth between the externally toothed gear and the drive-side internally toothed gear, and is outputted from the drive-side internally toothed gear. The rotation outputted from the drive-side internally toothed gear is reduced at a speed ratio of less than 50 in relation to the rotation inputted to the wave generator.
Patent Documents 2 and 3 disclose strain wave gearings having wave generators that have two rows of ball bearings. This type of wave generator is configured from a rigid plug having an ellipsoidally contoured outer-peripheral surface, and two rows of ball bearings fitted to the outer-peripheral surface. The flexible externally toothed gear is pressed radially outward by the two major-axis end portions of the outer-peripheral surfaces of the ellipsoidally flexed outer races of the ball bearings, and the meshing of the flexible externally toothed gear with respect to the first and second rigid internally toothed gears is sustained.